I've heard of it, I've seen it done, but I'm an Irrigation Contractor not a Landscaper, or a concrete guy. Do you mean gravel, or rock? I know that if you did pour over soil, it would still have to be compacted, and very stable to start with.

If you live in an area in which the ground freezes and thaws in the winter it would be a big mistake.
The high moisture content often found in soil causes it to expand greatly when it freezes & then the slab will heave and crack.

Rock or gravel is used as a proper base because it allows water to quickly drain and moisture to dry outward.

yes we used 2 inches of rock over the dirt and will pour 4 inches of cement.... I wanted 4 inches of rock and 5 1/2 inches of cement but contractor says we dont need that much because it is a patio and not a driveway.. Im just concerned about the winters here in western Pa.. I know the first year is crucial so that the cement dont crack.. someone told me to also get some hay or straw and leave it on the corners of the patio after the cement has dried to cure the patio.. anyone hear of that? thanks for the feedback